The expanding need for energy combined with the depletion of known crude oil reserves has created a serious need for the development of alternatives to crude oil as an energy source. One of the most abundant energy sources, particularly in the United States, is coal. Estimates have been made which indicate that the United States has enough coal to satisfy its energy needs for the next two hundred years. Much of the available coal, however, contains significant amounts of inorganic ash forming minerals, such as quartz and clay, and sulfur compounds, such as pyrites and organic compounds in admixture with the hydrocarbonaceous portion of the coal, which create serious pollution problems when burned. The amount of sulfur and ash forming mineral components in coal varies. However, virtually all types of coal contain such impurities and potential pollutants to some degree entrapped within the coal as mined. As a result, expensive pollution control equipment is usually required as part of any installation using coal as a fuel. The added cost of this equipment seriously detracts from and restricts the use of coal as an energy source.
To overcome the pollution problems associated with the combustion of coal, techniques have been developed for converting coal into liquids or gases from which the potential pollutants, e.g., sulfur, can be removed. For example, coal can be gasified into methane, water gas, and other combustible gases whereby the mineral matter contained in the coal is substantially removed during the gasification process. The sulfur containing pollutants, however, still remain in the resultant gaseous products and must be removed from these products by a separate processing step.
U.S. Pat. No. 3,850,738 issued to Stewart, Jr. et al provides another example of the conversion of coal to more valuable products. In this process, coal is contacted with water at high temperatures and pressures to thermally crack the hydrocarbonaceous material in the coal into aralkanes, gaseous hydrocarbons and undissolved ash.
Another technique for increasing the availability and use of raw coal involves the comminution of coal into a fine particle size in an effort to separate the coal into discrete component parts. One method of comminution, known as chemical comminution is illustrated in U.S. Pat. No. 3,850,477 issued to Aldrich et al involves weakening the intermolecular forces of the coal particles by anhydrous ammonia or other suitable chemicals.
Another method of comminution involves mechanical comminution or grinding. In this method, the grinding is effected by ball or jet milling or any other techniques wherein the coal particles impinge against or are contacted with a solid obstruction. Jet milling, for example, involves entraining coal particles in a gas stream at high velocity and directing the gas stream against a solid obstruction. Examples of jet milling are shown and described in Switzer, U.S. Pat. No. 3,973,733 and Weishaupt et al, U.S. Pat. No. 3,897,010. Specific examples of such jet milling devices include the "Micronizer" brand fluid energy mill manufactured by Sturtevant Mill Company and the "Jet-O-Mizer" fluid energy reduction mill produced by the Fluid Energy Processing and Equipment Company. These devices are described in an article, R. A. Glenn et al, A Study of Ultra-fine Coal Pulverization and its Application, pp. 20, 90 (October 1963), distributed by the National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Road, Springfield, Va. 22151. Mechanical comminution techniques are frequently used, for example, to provide feed coal to a gasification reactor.
Ball milling, jet milling and other mechanical impingement techniques involve relatively crude forms of comminution. First, and most importantly, these techniques do not comminute selectively; that is, they comminute the ash forming minerals as well as the valuable hydrocarbon portion of the coal. Another disadvantage is that the mechanical or grinding techniques do not separate or scission the hydrocarbonaceous matter within the coal from the mineral constituents of the coal. That is, ash forming minerals generally remain physically attached to the hydrocarbonaceous material in the coal, after milling, to a considerable extent. The minerals thus cannot be removed from the desired hydrocarbonaceous particles. In addition, organic forms of sulfur remain chemically bonded in the hydrocarbon. As a result, it is difficult to isolate the hydrocarbon from the pollutants. Second, these techniques are limited in their degree of size reduction. Ball milling and jet milling and other mechanical impingement techniques cannot effectively comminute coal, for example, to a mean particle size of less than about 2 microns.sup.1 because of the inherent elasticity of the coal. FNT .sup.1 As used herein, a micron is equivalent to a micrometer or 10.sup.-6 meter.
A third comminution method involves the explosive comminution of coal. This method, generally used with permeable, porous or microporous, friable solid materials, involves creating strong internal stress within the solid by forcing a fluid into the pores and/or micropores of the solid material at elevated temperature and/or pressure and then subjecting the material to rapid depressurization. The fluid within the pores and micropores thus expands very rapidly, thereby rupturing or exploding the coal into smaller particles.
The explosive comminution of solid materials has been investigated in connection with various fluids, temperatures, pressures, and operating designs. Singh, U.S. Pat. No. 2,636,688; Kearby, U.S. Pat. No. 2,568,400; and Yellott, U.S. Pat. No. 2,515,542 teach the use of gases such as air or steam as the comminuting fluid in connection with pressures between about 15 and about 750 pounds per square inch absolute (psia) and temperatures below the softening point of the coal. Schulte, U.S. Pat. No. 3,342,498; and Schulte, U.S. Pat. No. 3,545,683 teach the use of gases such as steam between about 500 and about 3,000 psia and between about 100.degree. and about 750.degree. F. not to comminute coal but to shatter ores. Lobo, U.S. Pat. No. 2,560,807; and Dean et al, U.S. Pat. No. 2,139,808 teach the use of a pressurized liquid such as water preferably below about 200 psia. Stephanoff, U.S. Pat. No. 2,550,390 teaches an explosive comminution reactor producing a product with a mean particle diameter of about 24 microns which is combined with a jet milling reactor to produce a final product with mean particle diameter of about 5 microns. Explosive comminution is also taught in Snyder, U.S. Pat. No. 3,895,760; and Ribas, U.S. Pat. No. 3,881,660.
Finally, the Jet Propulsion Laboratory (JPL) in Pasadena, Ca. has also conducted research on the feeding of coal into high pressure reactors. This research involves plasticizing solid coal at high temperatures and pressures, then screw extruding the resultant mass at high pressure through a nozzle. Fine particles are, as a result, sprayed into a reactor. This work is decribed in "Technical Support Package on Screw-Extruded Coal Continuous Coal Processing Method and Means," for NASA Tech. Brief, Winter 1977 (updated April 1978), Vol. 2, No. 4, Item 33, prepared by W. P. Butler.